1. Field of the Invention
The present invention relates to an electric power steering system which assists a steering operation by applying a driving force generated by an electric motor to a steering mechanism of a motor vehicle.
2. Description of Related Art
Electric power steering systems are conventionally available, which perform a steering assist operation by transmitting a torque (steering assist force) generated by an electric motor to a steering mechanism of a motor vehicle. In such an electric power steering system, an electric current command value, for example, is determined in accordance with a steering torque applied to a steering wheel and a vehicle speed. The electric motor is feedback-controlled on the basis of the electric current command value thus determined, whereby the steering assist force is properly applied to the steering mechanism.
If the motor vehicle is driven for a long time with the steering assist operation being frequently performed by the electric power steering system, a relatively great electric current is continuously applied to the electric motor. This may overheat the electric motor to an abnormally high temperature, causing a malfunction of the electric motor. A conventional approach to this problem is to take measures for protection of the motor, for example, by providing a heat sensing element within a housing covering the electric motor to limit the electric current command value on the basis of a temperature sensed by the heat sensing element.
To mount the heat sensing element within the housing of the electric motor, lead lines extending from plus and minus terminals of the heat sensing element should be led out of the housing, and gaps between the lead lines and the housing should be sealed for prevention of intrusion of moisture and the like into the motor. Therefore, the mounting of the heat sensing element in the conventional electric power steering system is troublesome, resulting in a cost increase.
One conceivable arrangement is such that the heat sensing element is mounted apart from the electric motor in a vehicle body rather than within the housing of the electric motor. More specifically, an ambient temperature around the electric motor is detected by the heat sensing element, and the temperature of the electric motor is calculated on the basis of the ambient temperature and the electric current applied to the electric motor. With this arrangement, however, the heat sensing element should be protected from moisture and the like, for example, by mounting the heat sensing element in a sealed case fixed to the vehicle body. This arrangement also entails the troublesome mounting of the heat sensing element.
It is an object of the present invention to provide an electric power steering system which requires less effort for the mounting of a temperature sensor such as a heat sensing element thereby to reduce the costs.
The electric power steering system according to the present invention comprises: a sensor for acquiring information necessary for controlling an electric motor; a board provided adjacent the electric motor and having a sensor signal processing circuit for processing a signal from the sensor; and a temperature sensor mounted on the board for acquiring temperature information necessary for controlling the electric motor.
According to the present invention, the temperature sensor is mounted on the board provided adjacent the electric motor. Therefore, the temperature sensor detects an ambient temperature therearound, and the temperature of the electric motor is calculated on the basis of the ambient temperature and an electric current applied to the electric motor, whereby a change in the temperature of the electric motor can indirectly be detected. Thus, the electric motor is prevented from being overheated to an abnormally high temperature, for example, by limiting the electric current flowing through the electric motor on the basis of the detection result obtained by the temperature sensor.
A temperature sensor which can automatically be mounted on the board with the use of an automatic mounting device may be used as the temperature sensor. This makes the mounting of the temperature sensor less troublesome. As a result, the production cost can be reduced.
The board is preferably housed in a water-tight case, and the case is preferably composed of a highly heat-conductive material. Where the board is housed in the case, it is necessary to lead a signal line out of the case for deriving a detection signal from the temperature sensor. The signal line is led out of the case, for example, through a water-tight grommet through which a signal line connected to the sensor signal processing circuit extends. Even with the temperature sensor mounted on the board, there is no need to alter the construction of the case which houses the board, and little effort is required for the lead-out of the signal line connected to the temperature sensor.
The temperature sensor is preferably connected to a ground line of the sensor signal processing circuit. This arrangement obviates a need for connecting one terminal of the temperature sensor to a lead line connected to an external ground line for grounding the terminal of the temperature sensor. Therefore, the number of required lead lines can be reduced by one as compared with the conventional construction in which the temperature sensor is provided within the electric motor, thereby reducing the costs of the electric power steering system.
The inventive electric power steering system preferably further includes a motor controlling circuit for controlling the electric motor on the basis of a signal processed by the sensor signal processing circuit and the detection signal of the temperature sensor. Thus, the electric current flowing through the electric motor can be limited on the basis of the detection result obtained by the temperature sensor for controlling the electric motor. This prevents the electric motor from being overheated to an abnormally high temperature.